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.\" aeb, various minor fixes
.TH SIGALTSTACK 2 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
sigaltstack \- set and/or get signal stack context
.SH SYNOPSIS
.B #include <signal.h>
.sp
.BI "int sigaltstack(const stack_t *" ss ", stack_t *" oss );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR sigaltstack ():
.ad l
.RS 4
.PD 0
_XOPEN_SOURCE\ >=\ 500
.\"    || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
    || /* Since glibc 2.12: */ _POSIX_C_SOURCE\ >=\ 200809L
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE
.PD
.RE
.ad
.SH DESCRIPTION
.BR sigaltstack ()
allows a process to define a new alternate
signal stack and/or retrieve the state of an existing
alternate signal stack.
An alternate signal stack is used during the
execution of a signal handler if the establishment of that handler (see
.BR sigaction (2))
requested it.

The normal sequence of events for using an alternate signal stack
is the following:
.TP 3
1.
Allocate an area of memory to be used for the alternate
signal stack.
.TP
2.
Use
.BR sigaltstack ()
to inform the system of the existence and
location of the alternate signal stack.
.TP
3.
When establishing a signal handler using
.BR sigaction (2),
inform the system that the signal handler should be executed
on the alternate signal stack by
specifying the \fBSA_ONSTACK\fP flag.
.P
The \fIss\fP argument is used to specify a new
alternate signal stack, while the \fIoss\fP argument
is used to retrieve information about the currently
established signal stack.
If we are interested in performing just one
of these tasks, then the other argument can be specified as NULL.
Each of these arguments is a structure of the following type:
.sp
.in +4n
.nf
typedef struct {
    void  *ss_sp;     /* Base address of stack */
    int    ss_flags;  /* Flags */
    size_t ss_size;   /* Number of bytes in stack */
} stack_t;
.fi
.in

To establish a new alternate signal stack,
\fIss.ss_flags\fP is set to zero, and \fIss.ss_sp\fP and
\fIss.ss_size\fP specify the starting address and size of
the stack.
The constant \fBSIGSTKSZ\fP is defined to be large enough
to cover the usual size requirements for an alternate signal stack,
and the constant \fBMINSIGSTKSZ\fP defines the minimum
size required to execute a signal handler.

When a signal handler is invoked on the alternate stack,
the kernel automatically aligns the address given in \fIss.ss_sp\fP
to a suitable address boundary for the underlying hardware architecture.

To disable an existing stack, specify \fIss.ss_flags\fP
as \fBSS_DISABLE\fP.
In this case, the remaining fields
in \fIss\fP are ignored.

If \fIoss\fP is not NULL, then it is used to return information about
the alternate signal stack which was in effect prior to the
call to
.BR sigaltstack ().
The \fIoss.ss_sp\fP and \fIoss.ss_size\fP fields return the starting
address and size of that stack.
The \fIoss.ss_flags\fP may return either of the following values:
.TP
.B SS_ONSTACK
The process is currently executing on the alternate signal stack.
(Note that it is not possible
to change the alternate signal stack if the process is
currently executing on it.)
.TP
.B SS_DISABLE
The alternate signal stack is currently disabled.
.SH RETURN VALUE
.BR sigaltstack ()
returns 0 on success, or \-1 on failure with
\fIerrno\fP set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
Either \fIss\fP or \fIoss\fP is not NULL and points to an area
outside of the process's address space.
.TP
.B EINVAL
\fIss\fP is not NULL and the \fIss_flags\fP field contains
a nonzero value other than
.BR SS_DISABLE .
.TP
.B ENOMEM
The specified size of the new alternate signal stack
.I ss.ss_size
was less than
.BR MINSTKSZ .
.TP
.B EPERM
An attempt was made to change the alternate signal stack while
it was active (i.e., the process was already executing
on the current alternate signal stack).
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lb
l l l.
Interface	Attribute	Value
T{
.BR sigaltstack ()
T}	Thread safety	MT-Safe
.TE

.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2009, SUSv2, SVr4.
.SH NOTES
The most common usage of an alternate signal stack is to handle the
.B SIGSEGV
signal that is generated if the space available for the
normal process stack is exhausted: in this case, a signal handler for
.B SIGSEGV
cannot be invoked on the process stack; if we wish to handle it,
we must use an alternate signal stack.
.P
Establishing an alternate signal stack is useful if a process
expects that it may exhaust its standard stack.
This may occur, for example, because the stack grows so large
that it encounters the upwardly growing heap, or it reaches a
limit established by a call to \fBsetrlimit(RLIMIT_STACK, &rlim)\fP.
If the standard stack is exhausted, the kernel sends
the process a \fBSIGSEGV\fP signal.
In these circumstances the only way to catch this signal is
on an alternate signal stack.
.P
On most hardware architectures supported by Linux, stacks grow
downward.
.BR sigaltstack ()
automatically takes account
of the direction of stack growth.
.P
Functions called from a signal handler executing on an alternate
signal stack will also use the alternate signal stack.
(This also applies to any handlers invoked for other signals while
the process is executing on the alternate signal stack.)
Unlike the standard stack, the system does not
automatically extend the alternate signal stack.
Exceeding the allocated size of the alternate signal stack will
lead to unpredictable results.
.P
A successful call to
.BR execve (2)
removes any existing alternate
signal stack.
A child process created via
.BR fork (2)
inherits a copy of its parent's alternate signal stack settings.
.P
.BR sigaltstack ()
supersedes the older
.BR sigstack ()
call.
For backward compatibility, glibc also provides
.BR sigstack ().
All new applications should be written using
.BR sigaltstack ().
.SS History
4.2BSD had a
.BR sigstack ()
system call.
It used a slightly
different struct, and had the major disadvantage that the caller
had to know the direction of stack growth.
.SH EXAMPLE
The following code segment demonstrates the use of
.BR sigaltstack ():

.in +4n
.nf
stack_t ss;

ss.ss_sp = malloc(SIGSTKSZ);
if (ss.ss_sp == NULL)
    /* Handle error */;
ss.ss_size = SIGSTKSZ;
ss.ss_flags = 0;
if (sigaltstack(&ss, NULL) == \-1)
    /* Handle error */;
.fi
.in
.SH SEE ALSO
.BR execve (2),
.BR setrlimit (2),
.BR sigaction (2),
.BR siglongjmp (3),
.BR sigsetjmp (3),
.BR signal (7)
